Seventeen countries, the European Commission, and eight major tech companies have signed up to the accord.

"The achievement is significant," said Muslim Association of Canterbury spokesman Anthony Green.

"It won't wipe out the emotion that drives these things, but if it helps to remove the platform, then that is to be welcomed."

But in a blow to the strength of the mandate, the United States has chosen not to sign despite extensive diplomatic efforts and the fact that a representative was in Paris at a parallel meeting of G7 digital ministers.

The call is still an unprecedented agreement between governments and all the major tech companies for ongoing collaboration to make the internet safer.

However, the White House will not sign the agreement amid US concerns that it clashes with constitutional protections for free speech.

Prime Minister Jacinda Ardern noted the United States' support for the call's principles.

The US reluctance to sign up was in part mitigated by the commitments of the tech companies that are mainly based there, she said.

Dialogue with the US was ongoing, she added.

Speaking as an individual, Green was disappointed that the United States refused to sign up to the agreement.

"American people should start to reflect seriously on the status of America as a significant outlier on so many issues ... and it should be a significant concern to us all," he said.

The Christchurch Call is the culmination of weeks of intensive work across many government departments, involving thousands of officials to draw up the document and garner global support just two months after the terror attack.

While it is a voluntary framework, it has been given additional heft after an endorsement from 55 investor funds that will use its $5 trillion in assets to push the tech companies to follow through on their pledges.

And five major tech companies have released a series of commitments, including regular publishing of transparency reports about detecting and removing terrorist or violent extremist content on their online platforms, to strengthen the Call to Action.

They also agreed to establish incident management teams to urgently respond to objectionable content.